1. Field of the Invention
This disclosure describes a data organization for CD-ROM disks that uses a symmetric structure, replication scheme, and frequency-based file placement techniques to improve CD-ROM access response time.
2. Description of Related Art
Digital record discs for recording information signals such as digitized audio signals, video or image signals have become quite common. Such digital record disks, e.g., so-called CDs (Compact Disks) have recently become used as ROM (Read Only Memory) devices for computers, so-called CD-ROMs.
On a CD, digital data intended for high density recording is recorded in the form of a spiral bit string on one side thereof and reproduced through reading the bit string by means of pickup means such as an optical pickup using laser beams. The optical pickup of the record disc reproducing apparatus follows the bit string by means of a tracking control system. Geometrically, a CD is 12 cm in diameter and 1.2 mm in thickness. Structurally, a CD is a transparent disk made of synthetic resin and covered with a metal thin film with circularly arrayed pits. These pits represent digital data as obtained by pulse-code-modulating (PCM) the analog acoustic information as picked up. The disk is rotated at a rotating speed in the range from approximately 500 to 200 rpm. A CLV (constant linear velocity) system is used for driving the disk. For playing back data from the disk, an optical pick-up (involving a semiconductor laser, a photo-electric conversion element, and the like) is linearly moved from the inner peripheral side to the outer peripheral side of the disk. A tracking system is used for moving the pick-up. A disk with a 1.6 .mu.m track pitch is capable of storing a great amount of information.
Thus, a great amount of data is recorded on the disk. The particular disk player used with this disk must search for the intended data from the large amount of data at the highest possible speed. The search function is therefore an essential feature of the disk player.
The CD player was originally developed to playback musical data. The large memory capacity of the CD, however, provides the new use of the CD as a CD-ROM reproduction apparatus. In this system, the CD is used as a ROM or a read only recording medium. Many types of digital data can be stored in the memory area typically used for storing audio data in conventional CD players. To record and reproduce the digital data, the CD-ROM reproduction apparatus employs a conventional record/playback format. Additionally, a new format is also used as described below.
In CD playback systems, analog audio signals of two channels (L and R) are sampled at 44.1 kHz. The sampled data is digitized and recorded onto the CD as 16-bit digital audio data. In CD-ROM reproduction systems, on the other hand, the 16-bit digital audio data of both channels is divided into two pieces of data, each including 8 bits (one byte data). The digital data is stored as a data unit (one block) of 2352 bytes. One block of data includes a sync pattern for checking the start of each block (12 bytes) and, a header address as the address data of the block (every 1/75 sec.), (r bytes), data for user (2048 bytes=2 k bytes), and data for data error detection and correction (mode I) or data for user (mode II) (288 bytes).
Mode I is used when data error cannot be corrected by the parity correction for both channels C1 and C2 in the audio data playback format. If Mode I is used, the error rate can be improved up to 10-.sup.12. This low error rate enables the CD to be used as the recording medium for a computer.
Mode II allows the operator to use the memory area of 288 bytes. When Mode II is used, the error rate is 10-.sup.9 at most, but the amount of usable data in one block is increased to: EQU 2048+288=2336 bytes.
One block of data is reproduced every 1/75 sec. In Mode I, this is 2K bytes of data per block. The data transfer rate of the CD-ROM reproduction system is 2K.times.75=150K bytes/sec.
Therefore, in the case of the disk which is used in the CD-ROM reproduction system and has a memory capacity of the data providing one-hour continuous playing on each side, the data recorded therein is EQU 150K bytes.times.60.times.60=540M bytes.
The memory capacity of the CD amounts to that of normal floppy disks of 500 to 1000 or 270,000 documents each containing 200 letters written therein. In this respect, the CD-ROM disk is a data recording medium featuring a low error rate and a tremendously large storage capacity.
The CD-ROM disk is a data recording medium. To process the data recorded, therefore, a host system or a host computer is required. In use, an operator operates a keyboard of the host computer, to cause the host computer to send a command to the CD-ROM reproduction apparatus. In response to the command, the reproduction apparatus searches for the address of the desired data. Following the search, the reproduction apparatus transfers the data specified by the searched address, back to the host computer.
Because CD-ROM reproduction apparatuses are used as a data memory apparatus, high speed data access is a desirable performance feature.
Some approaches to data access of CD-ROMs are described in U.S. Pat. No. 4,937,803 issued Jun. 26, 1990 to Nakane entitled "TRACK ACQUISITION APPARATUS AND METHOD WITH SPECIFIED VELOCITY PATTERN" which discloses data reading device for use in a disk player including a pickup device for reading data from a disk, a pickup transfer motor for moving the pickup across the disk in a radial direction in response to a speed control signal, a pickup transfer speed detector for detecting the radial speed of the pickup and generating a speed signal representative of the speed, a target position signal generator for generating a position signal corresponding to a desired radial position on the disc, a microcomputer for calculating the location of the pickup with respect to the disc and generating a location signal corresponding to the location, a comparator for generating a distance signal representing the distance between the location of the pickup and the desired position using the position signal and the location signal, nonlinear attenuator for attenuating the speed signal in a nonlinear manner based upon the value of the speed signal and a pickup drive circuit for generating the speed control signal from the distance signal and the attenuated speed signal and applying the speed control signal to the pickup transfer motor for moving the pickup.
In U.S. Pat. No. 4,949,323 issued Aug. 14, 1990, to Yoshida entitled "DISK REPRODUCTION APPARATUS AND METHOD FOR HIGH SPEED ACCESSING OF DESIRED INFORMATION DATA BY CHECKING ADDRESS DATA" a system is disclosed wherein, when receiving address data of a target position from a host computer, a control unit reads the present address data of a pick-up using a subcode Q detector or a header detector. The control unit computes a difference between the present address data as read out and the address data of the preceding frame (one block) already read out and stored in RAM. The control unit then checks if the difference is within a preset value. If it is not, the control unit computes a difference between the address data at the target position as specified by the host computer and the present address. The control unit then checks to see if the difference is within a preset value. If it is, the present address data is correct it is used in the data access operation.